In recent years, many fabrics have been developed which the various manufacturers claim are both waterproof and water vapour permeable. These materials are commonly described as waterproof breathable fabrics (WBFs), and they normally incorporate a continuous polymer membrane. The membrane may be in the form of a thin coated layer applied directly to the fabric, or as a pre-cast film subsequently bonded to the fabric with an adhesive layer. The direct coatings may comprise one complete layer or more usually a series of different layers, for example, a base coat or tie coat directly attached to the fabric, one or more intermediate coats, and an outermost or top coat. The direct coating may therefore have the same polymer composition throughout, but more usually comprises a series of different polymer compositions applied by successive coating operations. In particular, the base coat and top coat usually have a substantially different polymer composition.
The polymers used in WBFs include poly(tetrafluoroethylene), polyester, polyamide and especially, polyurethane. The complete coatings, separate coated layers, pre-cast films and adhesives may be formed from microporous polymers or hydrophilic polymers, or various combinations thereof may be used in the manufacture of WBFs. A useful reference book for this technology including descriptions of manufacturing techniques and machinery is “New Materials Permeable to Water Vapour”, Dr Harro Träubel, Springer-Verlag (Berlin), 1999.
The major use of WBFs is in the waterproof clothing area, although these materials are also used in footwear and industrial clothing as well as some other non-apparel applications. The water vapour permeability of the polymer membrane is sufficient to allow sensible and insensible perspiration to diffuse away from the body.
In some applications, the fabrics have to meet recognized Standards and Specifications lo in terms of liquid water resistance or waterproofness and water vapour transmission properties. For example, fabrics complying with the requirements of “EN 343:2003 Protective clothing—Protection against rain” are often specified for industrial workwear. In this and similar Standards, the breathability of a fabric is evaluated by measuring its water vapour resistance (Re). Fabrics can be classified, partly on the basis of their Re values as Class 1 with an Re of greater than 40 m2 Pa/W, Class 2 with an Re of between 40 and 20 m2 Pa/W, or Class 3 with an Re of less than 20 m2 Pa/W. Waterproofness is assessed by progressively increasing the liquid water pressure applied to a fabric sample until one droplet penetrates through (hydrostatic head test). The current requirements for Classes 1, 2 and 3 are minimum hydrostatic head resistances of 8, 8 and 13 kPa, respectively. The technical performance of the fabric therefore increases from Class 1 to Class 3. This type of classification and performance requirement is relevant for the present invention and many of the applications described herein.
It should be noted that Re values are more valuable than alternative measurements of water vapour permeability or transmission, because they are additive quantities. This means that the total water vapour resistance of a clothing assembly, film laminate or multilayer film can be approximated by summation of the resistances of the individual components. The general formula for resistance is thus given by:
      R    =                  ∑        1        n            ⁢              R        i              or      R    =                  R                  i          ,          1                    +              R                  i          ,          2                    +                        R                      i            ,            3                          ⁢                                  ⁢        …            +              R                  i          ,          n                    where R is the overall resistance of an assembly, n is the number of layers and Ri is the resistance of each individual layer. Normally, the ordering of layers is not important, i.e. the total resistance of layers stacked in order a,b,c,d is the same as that for other orders such as d,c,b,a or c,d,a,b. This concept is already widely used, for example, for measuring or calculating the thermal resistance of clothing and bedding, the electrical resistance of circuits and the gas or vapour resistance of packaging materials.
A major class of WBF incorporates hydrophilic (water-loving) polyurethane-containing coatings. These hydrophilic polyurethanes are usually segmented or block copolymers comprised of alternating hard and soft segments in their molecular backbone. Hydrophilic polyurethanes often contain 30-60% by weight of poly(ethylene oxide) (PEO) soft segments having the formula—[(CH2)2O]n—where n is an integer representing the average number of ether monomer units in the segment. Typically n ranges from 10 to 50. These interconnected PEO soft segments are usually introduced by reaction of diisocyanates with poly(ethylene glycol)s of various molecular weights which binds the PEO segments into the backbone of the polyurethane chains. The value for n can be estimated from the average molecular weight of the poly(ethylene glycol) which in turn is estimated from standard measurements of hydroxyl content. The hard segments in PEO-containing polyurethanes are usually made from reaction of residual isocyanate groups with short chain diols such as butan-1,4-diol. Other technologies exist for incorporating PEO in pendant groups or terminal groups on the molecular chains.
It should be stated that PEO is water-soluble and is prone to crystallization due to the stereoregularity and close-packing ability of the molecular chains. Therefore the weight proportion and average chain length (or n value) of these PEO segments within the polymer must be confined within a certain range in order to ensure that the polyurethane coating or film remains insoluble in use or does not act as a hydrogel.
The hydrophilic polyurethane coating or film is a solid (i.e. non-microporous) material and therefore does not allow liquid water droplets such as rain to penetrate. However, individual molecules of water vapour may pass through the polyurethane structure by a molecular diffusion action. Thus, these coatings can fulfil the primary function of a membrane used in WBFs.
The diffusion of water vapour through hydrophilic polyurethanes is a complex process, but is thought to involve a number of interactive mechanisms, principally:    1. Movement through pre-existing holes in the polymer structure. Movement via this mechanism is dependent on the number and distribution of pre-existing holes in the material which is in turn dependent on factors such as the physical packing of the molecular chains, free volume and density.    2. Movement via transient holes in the polymer structure. This mechanism clearly depends on the ability of transient holes to form in the material, which in turn relates to segmental chain mobility and cohesive energy between adjacent molecules.    3. Movement assisted by a hydrogen bonding mechanism. This involves the formation of weak temporary bonds between the diffusing water molecules and a series of chemical groups strategically located in the polymer network such that continuous pathways of hydrogen bonding sites exist throughout the coating or film. This can be envisaged as provision of molecular stepping stones.    4. A swelling mechanism, where the influx of water molecules is so great that the molecular chains are forced apart, which can further accelerate the diffusion process. This swelling mechanism can involve substantial uptake of water leading to increases in the surface area and volume of the membrane, which in turn may have a deleterious effect on certain properties. This is the main feature that distinguishes so-called hydrophilic polymer coatings from microporous and low water vapour permeable (both non-swelling) types.
The driving force for water vapour diffusion is provided by the difference in water vapour pressure at the surfaces of the coating. Vapor flows from the region of higher water vapour pressure to the region of lower water vapour pressure. The rate of water vapour diffusion through the membrane is directly proportional to the pressure difference between its two surfaces, and inversely proportional to its thickness. A dynamic equilibrium exists when the vapour pressure at both surfaces eventually becomes equal.
Soft segments containing polyether moieties may be considered for water vapour diffusion involving mechanisms 1-3. The ether group imparts flexibility in the chain and provides hydrogen bond acceptor sites. Usually, however, the diffusion rate of water vapour through these types of polyurethane is relatively slow and insufficient, for example, to provide coated fabrics or laminates which meet the Class 2 or 3 requirements of EN 343:2003 for Re. The exceptions are polyurethanes containing typically 30-60% by weight of PEO, which can be used for textiles meeting the high performance level requirements. This is because PEO has an exceptional affinity for water and will also participate strongly in mechanism 4.
PEO provides a unique capability for hydrophilic transport of water molecules since its stereochemistry mimics that of water itself, for example in terms of the bond angles, bond lengths and hydrogen bond strengths. In particular, the distances between adjacent oxygen atoms in the PEO molecular chains are identical to those in polymeric water clusters. Hydrogen bonding in the two species is therefore entirely complementary.
Other simple polyether moieties such as poly(methylene oxide), poly(acetaldehyde), poly(propylene oxide), poly(trimethylene oxide) or poly(tetramethylene oxide) have a different stereochemistry that is significantly less compatible with that of water. In fact, these other polyethers are insoluble in water. Moreover, polyethers such as poly(acetaldehyde) and poly(propylene oxide) transmit water vapour at lower rates because the bulky methyl group adjacent to the ether oxygen atom impedes the movement of water molecules according to mechanism 3. These polyethers with pendant methyl groups are also less able to exhibit close packing of the molecular structure, which leads to poorer physical properties. Thus, PEO-containing polymers are currently the materials of choice for solid, water vapour permeable polyurethane coatings and films.
With hydrophilic PEO-based polyurethanes, water vapour transport takes place mainly by mechanisms 1-3 under surrounding conditions of low to moderate water vapour pressures or relative humidities. The swelling mechanism 4 is initiated at much higher water vapour pressures, at higher relative humidities for example over 95%, or especially when liquid water comes into contact with the coating or film. Under these latter conditions, the polyurethane will absorb water and swell.
In the swollen state, the diffusion constant for the system and hence the rate of water vapour diffusion through the PEO-based polyurethane can increase by a factor of 3 or more. It is thought that a favorable transport process in this swollen state involves two water molecules that can hydrogen bond to each ether oxygen atom of PEO, whilst a third water molecule forms a temporary bridge between them or a bridge across to a bound water molecule on an adjacent PEO segment. Vapor flow occurs by transfer of these temporarily bound water molecules along PEO chains. However, due to the extreme hydrophilic nature of the PEO soft segment, it is not always possible to restrict the water-uptake of the coating to three molecules per ether oxygen atom and the material will take up even greater amounts of water. When this occurs, water molecules may cluster together and condense within the polymer structure causing the polymer to further swell.
The degree of swelling is normally controlled by careful selection of the hard segment constituents that constrain the PEO soft segment, and optionally by crosslinking. Hydrophilic PEO-containing polyurethanes may have water uptake capacities of 30-100% w/w, but more typically about 50% w/w in a top coat composition. As the coating or film dries out, the polyurethane usually returns to its original dimensional state and physical properties.
Swelling is therefore a normal and controlled feature of hydrophilic PEO-containing polyurethane films and coatings, and it can usually be accommodated in the specific end-application. In some applications, however, this potential swelling is unacceptable and precludes the use of hydrophilic PEO-containing polyurethane coatings. If the membrane is visible, for example in unlined garments or as external coatings on waterproof garments or footwear, contact with droplets of rainfall or condensation may cause uneven areas of swelling that result in unsightly blistering or wrinkling effects. It may be particularly noticeable in unsupported membranes or in coatings attached to dimensionally less stable fabrics such as loosely woven fabrics, scrims, pile fabrics, warp or weft knitted fabrics and nonwovens, which are used in many WBF applications.
There is therefore a recognized need for water vapour permeable, waterproof polyurethane coatings and films which have substantially reduced water uptake, or better still no propensity for swelling.